采用分子动力学方法研究氯化钠水溶液的微观结构

溶液的微观结构对溶液的宏观性质具有决定性影响，团簇的存在和溶液不均性的认知是该领域研究的重要进展之一，也是关注的热点.为了考察溶液的微观结构，本文采用分子动力学方法对浓度适中的氯化钠水溶液进行了模拟，获得了各原子(离子)间的径向分布函数和溶液的瞬态图像.通过对比分析Na~+-Cl~-和Cl~--Na~+的径向分布函数，并结合已有研究结果，表明该径向分布函数上第三峰也来源于离子对，发现氯化钠水溶液存在着第三种离子对；上述径向分布函数上的三个峰分别与直接接触离子对、部分间隔离子对和完全间隔离子对相对应，并给出了这三种离子对的瞬态图像.三个时刻下系统瞬态图像的分析结果表明，溶液中存在水分子连续分布的区域和由离子与水分子共同构成的离子团簇.各瞬态图像中水分子连续分布的最大区域的平均尺寸至少为1.43 nm,溶液中部分间隔离子对的比例最大，约为0.68.在同时考虑直接接触离子对和部分间隔离子对时，三个时刻中最大团簇尺度的平均值为1.44 nm,该值约为部分间隔离子对最概然尺寸的3倍；团簇尺寸随团簇中离子数的增加而增大，在团簇中离子数较大时，满足无规行走(自由连接链)模型的结果.这些说明氯化钠水溶...

Microstructures of sodium chloride aqueous solution investigated by molecular dynamics method

The microstructures of solutions have a major impact on their macro-properties and the cognition of the existence of clusters and heterogeneity in solutions is not only one of the important advances but also the focus in this field. In order to investigate the microstructures of solutions, the simulation of sodium chloride aqueous solution with moderate concentration is carried out by molecular dynamics method and the radial distribution functions (RDFs) between atoms (ions) and the transient images of the solution are therefore given. By comparing the RDFs of Na+-Cl- and Cl--Na+ and combined with the previous research results, it is shown that the third peak of their RDFs also comes from the ion-pair and the third kind of the ion-pair is found in the aqueous solution of sodium chloride; i.e., the three peaks of the above RDFs are respectively related to the directly-contacting ion-pairs, the partially-spaced ion-pairs and completely-spaced ion-pairs, meanwhile, which transient images are also given. The analysis results of the system transient images at three moments show that there are regions with continuous distribution of water molecules (RW) and ionic clusters composed of ions and water molecules in the solution. The average size of the largest RW in each transient images is at least 1.43 nm and the proportion of partially-spaced ion-pairs is the greatest, which is about 0.68. The average value of the maximum cluster size in each transient images is 1.44 nm when both directly-contacting ion-pairs and partially-spaced ion-pairs are considered, which is about 3 times the most probable size of partially-spaced ion-pairs. The cluster size increases with the rising ionic number of the one, which meets the results of the random walk (freely jointed chain) model. The above results show that there are self-concentration effects and micro-heterogeneity in sodium chloride solution and suggest that self-concentration effects also exist in ionic solution. The components of the above regions and clusters are respectively similar to those of ice and NaCl•2H2O crystal formed by sodium chloride aqueous solution, which enlightens people to think about the relationship between the two kinds of crystals and the above regions and clusters.